Dispersion strengthening of metals and alloys



United States Patent 3,434,830 DISPERSION STRENGTHENING 0F METALS AND ALLOYS Nicholas J. Grant, Leslie Road, Winchester, Mass. 01890 No Drawing. Filed Oct. 22, 1965, Ser. No. 502,469 Int. Cl. B22f 1/00 US. Cl. 75-206 1 Claim ABSTRACT OF THE DISCLOSURE The invention relates to the production of dispersion strengthened metals from finely divided matrix-forming material while inhibiting the pyrophoric and presintering tendencies of the finely divided material during the initial stages of thermal processing. The method employed comprises starting with finely divided reducible metal oxide whose negative free energy of formation of the oxide is less than 80,000 calories per gram atom of oxygen at about 25 C. and mixing with it a solution having dissolved therein a refractory oxide-forming salt which decomposes to a refractory oxide coating whose negative free energy of formation is at least 100,000 calories per gram atom of oxygen at about 25 C. The metal oxide powder with the refractory oxide coating is then reduced at an elevated temperature to form very fine metal power having the refractory oxide on the surface thereof while inhibiting the pyrophoricity and the presintering tendency of the powder.

This invention relates to the production of high strength metals and alloys and, in particular, to a method of dispersion strengthening metals and alloys characterized by improved room and high temperature strength properties.

Ever since the discovery of SAP (sintered aluminum powder), much work has been done in studying the principles of dispersion strengthening and applying them to new metal systems. This type of strengthening has been studied primarily by means of powder metallurgy techniques, one method comprising using metal powder as the matrix-forming material which is blended uniformly with finely divided refractory oxide particles, such as A1 0 and the mix thereafter consolidated and hot worked to the desired shape. Studies with this system have indicated that the finer the metal powder and the disperse phase, the higher are the strength properties in the final product.

However, in working with systems of the foregoing type particularly where the average size of particles of metal are extremely fine, for example, below several microns, such as up to about 2 microns and even below one micron, considerable care must be taken in handling the metal powder during the initial stages of processing, otherwise the fine metal powder, because of its pyrophoricity, oxides exothermically, which generally results in the final product exhibiting somewhat inferior properties. A method for handling such fine metal powders is disclosed in U.S. Patents Nos. 3,175,904 and 3,176,386.

I have now discovered a. method of processing very fine materials while avoiding the problems generally inherent in working with fine materials. I find that by employing the method of the invention, I can produce dispersion strengthened materials having superior high strength properties. In addition, I have found that the method also enables working with extremely fine materials ice While avoiding premature sintering during the early stages of processing.

It is the object of this invention to provide a powder metallurgy method of producing dispersion strengthened metal systems by using extremely finely divided starting ingredients.

Another object is to provide dispersion strengthened metals and alloys characterized by improved strength properties.

These and other objects will more clearly appear from the following disclosure.

In its broad aspects, the invention is directed to the production of dispersion strengthened metals (it being understood that the term metal or metals also includes alloys) from finely divided matrix-forming materials while inhibiting the pyrophoric and presintering tendencies of the finely divided material during the initial stages of thermal processing. By utilizing the method of the invention, I find that I can work with finely divided matrixforming materials of average size less than one micron, for example in the neighborhood of about one-half micron or less and not be concerned with pyrophoricity.

Broadly speaking, the method comprises providing finely divided particles of at least one reducible metal oxide whose negative free energy of formation is less than 80,000 calories per gram atom of oxygen at about 25 C.; providing a solution of a soluble decomposable refractory oxide-forming salt in a non-residue leaving solvent, the oxide formed from the salt being one whose negative free energy of formation is at least about 100,000 calories per gram atom of oxygen at about 25 C.; and then mixing the reducible metal oxide with the solution. Thereafter the solvent is removed and the metal oxide dried so that the metal oxide particles have associated therewith the said decomposable salt, for example generally as a coating. The metal oxide mixture is then heated to an elevated temperature to decompose the refractory oxideforming salt, and the metal oxide then reduced by heating it in a reducing atmosphere at an elevated temperature to convert the particles of metal oxide to finely divided metal having dispersed uniformly therethrough finely divided metal oxide.

An advantage of the foregoing method is that the thin film of refractory oxide generally surrouding or separating the metal oxide after decomposition of the salt enables carrying out high temperature reduction without substantially undergoing agglomeration and growth of the subsequently reduced metal particles. This was noted in particular when a mixture of submicron nickel oxide and molybdenum oxide powders was blended with an alcohol solution of thorium nitrate. Even after reduction treatments of the order of 1000 C. to 1200 C., there was substantially no sintering observed. The coating of thorium oxide was additionally advantageous in that it enabled hydrogen to diffuse through it while inhibiting pyrophoricity of the newly reduced metal particles formed during hydrogen reduction.

In carrying out the invention, one may start with metal oxide particles of sizes exceeding five or ten microns, or higher, as such particle sizes can be easily broken down during wet mixing in a ball mill. The metal oxide employed may be derived from any ductile metal having a melting point exceeding 250 C., but usually exceeding 650 C. Examples of matrix-forming materials are metal oxides of such metals as the copper group metals Cu, Ag,

Au and Cu-base, Ag-base and Au-base alloys; Pb and Pbbase alloys; the iron group metals Fe, Ni, Co and Fe-base, Ni-base and Co-base alloys; the platinum group metals and alloys thereof; and other matrix-forming metals, just so long as the oxides of the matrix-forming metals have a negative free energy of formation below about 80,000 calories per gram atom of oxygen at about 25 C.

Examples of copper group alloys are: 90% Cul% Ni, 80% Cu-% Ni, 70% Cu30% Ni, 70% Cu-30% Au, 90% Ag-l0% Cu, etc.

Examples of iron group alloys are 64% Fe-36% Ni, 31% Ni-4 to 6% Co-balance Fe, 54% Fe46% Ni, 90% Fe-l0% Mo or W, 90% Ni10% M0 or W, and similar alloys.

In addition to the foregoing, other metals and alloys may be employed so long as the negative free energy of their oxides is below about 80,000 calories per gram atom of oxygen at about C.

With regard to the decomposable refractory oxideforming salt, those salts are preferred which are soluble in non-residue leaving solvents, such as water, or alcohol and other organic solvents, and which salts decompose to a refractory oxide whose negative free energy of formation is at least about 100,000 calories per gram atom of oxygen at about 25 C. and, more preferably, at least about 120,000 calories. The solvent should be one which evaporates below the decomposition temperature of the salt and does not leave a residue. The soluble salt is preferably one whose decomposition temperature does not exceed the melting point of the reduced metal. I find it desirable that the decomposition temperature of the salt not exceed about 800 C. and more desirably not exceed about 375 C.

Examples of refractory oxide-forming salts which decompose to form dispersions of refractory oxides are:

2 3 2)3; 2 4)a' 2O 2NH4NO3CC(NO3)34HZO; Ba(NO BC(NO3)23H2O Ca(NO '4H O; a)s 2 2 3 2)2.' 2

SI(NO3)24H2O 5TiO2N205 etc. Certain of the chloride salts may be employed provided the prevailing conditions are such as to favor formation of oxides. Thus, the salts may comprise nitrates, oxalates, acetates, chlorides and the like. The foregoing salts are at least to some extent soluble in water, alcohol or both. The refractory oxides formed by the decomposition of these salts have negative free energy of formation of at least about 100,000 calories per gram atom of oxygen at about 25 C.

I find it advantageous to use thorium nitrate as the source of ThO as I find that rather high strength properties can be obtained with it. In addition, thorium nitrate is easy to work with because of its high solubility in alcohol.

As illustrative of the invention, the following examples are given:

Example 1 A series of nickel-thoria compositions were produced containing approximately 3, 6 and 9 vol. percent of ThO respectively. In producing the compositions, a batch of nickel oxide powder of about 0.6 micron particle size is mixed with an ethyl alcohol solution of thorium nitrate to form a slurry containing sufficient thorium corresponding to the desired volume percent of thorium oxide and the mixture then ball milled in a nickel-lined mill for about 24 hours, care being taken throughout the milling operation to be sure that adequate alcohol is added to maintain a suitable viscosity. Upon completion of the milling, the mixture is subjected to vacuum treatment at about 80 C. to remove the alcohol. The resulting aggregate is then screened to yield a friable aggregate of between plus 20 and minus 4 mesh of high porosity.

The friable aggregate is then subjected to vacuum decomposition at a temperature of about 600 C. to 700 C. for about 1 to 3 hours to produce a nickel oxide powder with the particles having associated therewith thorium oxide. The decomposed powder mixture is then subjected to hydrogen reduction at a temperature ranging from about 600 C. to 800 C. using a constant flow of hydrogen at a rate of 6 liters per minute for time intervals ranging up to about 24 hours, after which the batches are cooled to room temperature. Because of the presence of thoria, sintering of the resulting metal powder product is greatly inhibited during the reduction treatment as is its pyrophoricity.

Example 2 In the production of dispersion strengthened nickelmolybdenum alloys, the procedure of Example 1 is employed. Quantities of 0.6 micron of nickel oxide and 0.5 molybdenum oxide (M00 are blended with an ethyl alcohol solution of thorium nitrate and ball milled as aforesaid, the amount of molybdenum oxide added being sufficient to provide several nickel alloys containing about 12% molybdenum and containing in addition approximately 3, 6 and 9 vol. percent of thorium oxide, respectively. As in Example 1, the starting mixture is vacuum dried, then decomposed to remove volatile nitrogen oxide gases, and finally reduced in hydrogen substantially as in Example 1.

The powders produced in Examples 1 and 2 exhibited a density of about 25% of true density. However, when hydrostatically compacted at about 35,000 p.s.i., the product had a density of about 60%. When compacted in a press at about 7,000 p.s.i., the product exhibited a density of about 50% of true density. Compacts produced by either of the foregoing methods were vacuum treated and then heat treated in hydrogen for the purpose of increasing the density of the compacts and to eliminate any water vapor or oxygen which may have been picked up in the handling of the powders. The compacts were then sealed in evacuated mild steel cans and the canned material hot extruded over the temperature range of about 954 C. to 1038 C. with preheat at these temperatures for about 1 to 2 hours. A total of 12 compositions were hot extruded. With regard to Alloy Nos. (4) to (6) and (10) to (12), these were hot extruded in two steps: the first step with a reduction ratio of 4:1, the second step with a reduction ratio of 33:1. Alloy Nos. (1), (2), (7) and (8) were hot extruded in a single step at a ratio of 12:1, while Alloy Nos. (3) and (9) were hot extruded at a ratio of 23:1.

Room temperature tension and creep-rupture tests at 982 C. (1800 F.) were conducted on the alloys in air. In addition, the extruded alloys were examined metallographically and were found to exhibit unusually uniform oxide dispersions. The average particle size of the thoria phase was obtained by electrolytic extraction by dissolving alloy chips in hot nitric acid and then measuring the size of the oxide by using X-ray broadening techniques. The compositions of the alloys and the room temperature strength properties are given in Tables I and II as follows:

TABLE I Alloy No. Composition T1102, v/o Size ThOz, A

1 Size in Angstrom Units.

As will be noted, an extremely fine disperse phase was obtained ranging in size from 180 to 390 angstroms (or 0.018 to 0.039 micron).

1 Ultimate tensile strength.

2 Yield strength, 0.2% offset.

Hardness in Rockwell units.

As will be apparent, very high room temperature strength properties were obtained.

Creep properties were obtained on some of the alloys expressed as stress to produce a 100-hour rupture life at 982 C. (1800 F.). The results of these tests are given as follows:

TABLE III Alloy No. Composition T1101, v/o Stress, p.s.i.

A Ni+15% M0 None 3, 000

For comparison purposes, Alloy No. A comprising a 15% nickel-molybdenum with no disperse phase was included in the test. As will be noted, as the Th0 content exceeds about 6 vol. percent, the 100-hour rupture stress of the dispersion hardened material is about four times (9) raised its 100-hour rupture stress to 9,000 p.s.i.

greater than Alloy No. A. Cold working of Alloy No.

With regard to dispersion hardened nickel Nos. (3) to (6), these specimens exhibited a 100-hour stress value at 982 C. of about 3,000 to 4,000 psi.

As will be readily apparent from the foregoing, high levels of room temperature strength properties and 982 C. creep-rupture strength can be achieved by starting with mixtures of sub-micron oxide of NiOThO NiO-MOO -ThO- and similar metal oxide systems. One of the advantages to be derived from the invention is that uniform alloy compositions can be produced by starting with ultra-fine ingredients in the oxide condition prior to reduction. In starting with nickel-base oxides, either dispersion hardened wrought nickel can be produced or dispersion hardened nickel-molybdenum alloys containing about to 25% or 5% to 15 molybdenum.

In producing a dispersion hardened nickel-copper alloy containing 67% nickel and 33% copper and say 10 vol. percent alumina, appropriate amounts of finely divided nickel oxide and copper oxide are blended together to which a solution of aluminum nitrate is added corresponding to the desired alumina content and the mixture then ball milled as in Example 1. Following vacuum drying, the mixture is subjected to a temperature to decompose the aluminum nitrate and to reduce the intimately mixed metal oxides in a stream of hydrogen. The reduced metals containing the fine dispersion of alumina are then further thermally processed as described hereinbefore to produce a dispersion strengthened wrought metal product. Similarly, systems of NiFe, Ni-W, Ni-W-Mo, Fe-W-Mo, Cu-Sn, Cu-Ag dispersion strengthened with such oxides as cerium oxide, beryllium oxide, rare earth oxides, and the like, may be produced.

As stated above, it has been found that by using the method of the invention, ultra fine starting materials may be employed while avoiding the problems normally associated with pyrophoricity. By obttining the disperse oxide phase through the decomposition of its corresponding salt, very fine refractory oxide dispersions are obtainable not exceeding substantially 2000 angstroms in size, and advantageously not exceeding 1000 angstroms, whether the oxide is Th0 A1 0 BeO, ZrO the rare earth metal oxides Ce O La O Y O and the like, MgO, CaO and other refractory oxides.

Stating it broadly, the invention provides a method of producing dispersion strengthened metals from finely divided matrix-forming materials while inhibiting the pyrophoric and presintering tendency of the finely divided material during the initial stages of thermal processing which comprises, providing finely divided particles of at least one reducible metal oxide Whose negative free energy of formation is less than 80,000 calories per gram atom of oxygen at about 25 C., uniformly dispersing about the particles of metal oxide a decomposable refractory oxide-forming salt whose negative free energy of formation of the oxide is at least about 100,000 calories per gram atom of oxygen at about 25 C., heating the oxide particles to an elevated temperature to decompose the refractory oxide-forming salt, and then subjecting the metal oxide to heating in a reducing atmosphere at an elevated temperature to reduce the particles of metal oxide to metal having finely divided refractory oxide particles uniformly dispersed therethrough.

In carrying the invention into practice, I find it advantageous starting with a solution of a soluble decomposable refractory oxide-forming salt in a non-residue leaving solvent, mixing the reducible metal oxide with the solution, removing the solvent and drying the reductible metal oxide, whereby the oxide particles have associated therewith a coating of the decomposable salt, heating the oxide particles to an elevated temperature to decompose the refractory oxide-forming salt, and then subjecting the metal oxide to heating in a reducing atmosphere at an elevated temperature to convert the particles of metal oxide associated with the refractory oxide to metal.

The finally reduced metal is characterized in that it can be hot worked to a wrought metal product.

The amount of decomposable salt employed should be sufficient to yield an oxide loading in the product ranging from about 2 to 25 volume percent and, more advantageously, from about 2 to 12 volume percent.

While it has been indicated that the refractory oxide referred to should have a negative free energy of formatio not at least about 100,000 calories per gram atom of oxygen at about 25 C., and preferably 120,000 calories, it is also preferred that the melting point of such refractory oxide be at least about 1600 C.

Generally speaking, the extrusion ratio employed in producing the wrought metal product may range from about 10 to l to 50 to l, and more preferably from about 20 to 1 to 30 to 1.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claim.

What is claimed is:

1. In a method of producing dispersion strengthened metals from finely divided matrix-forming material while inhibiting the pyrophoric and presintering tendencies of said finely divided material during the initial stages of thermal processing which comprises, providing finely divided particles of at least one reducible metal oxide of size not exceeding about 1 micron whose negative free energy of formation of the oxide is less than 80,000 calories per gram atom of oxygen at about 25 0., providing a solution of a soluble decomposable refractory oxide-forming salt in a non-residue leaving solvent, said refractory oxide formed from said salt being one whose negative free energy of formation is at least 100,000 calories per gram atom of oxygen at about 25 C., and being sufficient to provide a volume loading of about 2 to 25%, mixing said reducible metal oxide with said solution, drying said metal oxide free of said solvent, whereby said oxide particles are thinly coated with said decomposable salt, heating said oxide particles to an elevated temperature to decompose said refractory oxideforming salt and leave a coating of refractory oxide on said particles, subjecting said metal oxide to heating in a reducing atmosphere of hydrogen at an elevated temperature whereby to cause said hydrogen to diffuse through the refractory oxide coating and reduce said particles of metal oxide to metal while the refractory oxide coating inhibits the pyrophoricity of the newly reduced particles, the size of the refractory oxide ranging up to about 1000 angstroms and then hot-working said particles into a wrought metal product.

References Cited UNITED STATES PATENTS 2,823,988 2/1958 Grant et al 75206 X 3,019,103 1/1962 Alexander et al. 75206 3,158,473 11/1964 Gatti 75206 3,175,904 3/1965 Grant et al. 75206 3,320,057 5/1967 Palmateer 75212 X FOREIGN PATENTS 932,461 7/1963 Great Britain.

CARL D. QUARFORTH, Primary Examiner.

R. L. GRUDZIECKI, Assistant Examiner.

US. Cl. X.R. 

